The story is set in the year 2071, when the entire Solar System has been made accessible through hyperspace gates. In 2022, an explosion of an experimental hyperspace gateway severely damages the Moon, resulting in a debris ring and meteor bombardments that eradicate a large portion of the Earth's population. As a result, many survivors abandon the barely habitable Earth to colonize the inner planets, the asteroid belt and the moons of Jupiter.

Mars has become the new central hub of human civilization with specks of the human population inhabiting other planets, and interplanetary crime syndicates exert influence over the government and the Inter-Solar System Police (ISSP), limiting their effectiveness. As a result, a bounty system similar to that in the Old West is established to deal with fugitives, terrorists, and other criminals; the bounty hunters involved are frequently termed "cowboys". The standard currency is the woolong, which is roughly equivalent to the present-day Japanese yen.